How to make multiple molds

Troy Mccombs

I have a negative and positive ultracal mold for a foam prosthetic appliance and was wondering how I go about making multiple copies of both. I have tried, but for whatever reason, they don't match. The negative and positive don't fit into each other when I make copies, and I can't figure out why. I'm molding what I have with aljasafe alginate and then casting in ultracal.

Paolo Giovani

Alginate shrinks fast enough to make for kind of imprecise molds, if I wanted to make mold copies I'd go for epoxy materials instead of ultracal, might be more costly per mold, but considered it becomes basically indestructible you spend still much less than doing many ultracal molds.
If you can't solve first phase of the copy with the alginate, maybe you could invest in some thixotropic silicone, make a thin skin with it, back it with ultracal and burlap, this way you shouldn't spend too much on materials for a temporary mold, and then you make your final, epoxy (or ultracal) mold. This way you can get precise molds, and if you need multiple copies of your molds it will become cheaper by making everything last much longer.

Hope this helped!

Troy Mccombs

Paolo Giovani said:

Alginate shrinks fast enough to make for kind of imprecise molds, if I wanted to make mold copies I'd go for epoxy materials instead of ultracal, might be more costly per mold, but considered it becomes basically indestructible you spend still much less than doing many ultracal molds.
If you can't solve first phase of the copy with the alginate, maybe you could invest in some thixotropic silicone, make a thin skin with it, back it with ultracal and burlap, this way you shouldn't spend too much on materials for a temporary mold, and then you make your final, epoxy (or ultracal) mold. This way you can get precise molds, and if you need multiple copies of your molds it will become cheaper by making everything last much longer.

Hope this helped!

Wow, thanks! Yeah, I knew alginate shrinks, but didn't think it shrunk that fast.  So you recommend silicone and epoxy or just one or the other?

Paolo Giovani

Well, since to my understanding you already have the mold, and you want to just copy it, I would make the mold-of-the-mold in the cheapest way, so with the silicone skin (even if you don't have undercuts hard on hard is reeeally hard and risky to separate, I'd be scared to lose both the original and the new shell, but maybe you're more courageous than me), and an ultracal shell would be good enough, and then make the final mold copies with epoxy, so they will last for many more castings than ultracal, on the long run they should become cheaper than the ultracal since you will need new ones so seldom.
I always see people saying alginate is much cheaper than silicone, but here in Italy at least the price difference is like, double or even less, but you need 3-4 times the thickness of brush on silicone to make an alginate mold, so imo it is not always true alginate is cheaper for temporary molds.

Eric S

This sounds like might help with your project, or some of his other videos on mold making: https://youtu.be/12UahLu-NYw?si=orf5dTdCGHv5g00u